Wood as a trim material in exterior building applications has many excellent qualities. It is a naturally occurring material that is durable and strong, yet with some flexibility. It can be machined using a saw, a lathe, a router or other common wood machining equipment. Wood surfaces can be decorated to give a variety of appearances desired by a consumer. However, it is also susceptible to adverse effects from exposure to sunlight and moisture. High quality lumber necessary for creating shaped articles for trim pieces has also become relatively scarce and expensive.
For these reasons, plastics have been used in a number of applications in the building industry as a wood replacement, particularly in exterior applications. Common, high volume exterior applications include cladding and windows for which the use of “vinyl” (polyvinyl chloride) is well known. The use of plastics as a wood replacement is also known in doors, decks, fences, and in other applications. However, intricate shaped articles, including decorative moldings, door jambs and the like, which can require round surfaces, sharp angles, inside corners, undercuts and other difficult to fabricate shapes, have not been readily replaced by plastics because the manufacture of intricate plastic shaped articles has not been cost effective. Generally, intricate shapes can only be produced by expensive fabrication methods, for example injection molding, precise profile extrusion, machining, or other techniques designed for expensive engineered parts. The required intricate shapes, with all of the functionality required, have not been readily and cost-effectively produced from low cost plastics by known low cost methods of processing using, for example, common wood working routers, planers (thicknessers), molders and the like.
Machining of some plastics is known and can be used to produce intricate shapes and dimensional tolerances that cannot be readily fabricated into plastic parts using other means. As summarized in the March/April 2008 on-line issue of Plastics Distributor® & Fabricator Magazine some of the challenges typically encountered during machining of plastics, such as polypropylene, include re-weldment or wrap-around of waste material on the cutting tool and difficulty in obtaining the desired surface finish. One suggested method for addressing these challenges includes using machine tools which produce larger chips, such as slow helix tools. In addition, because of the sometimes gummy nature of polypropylene and the inherent heat generated by the cutting action during machining, it is recommended that high-speed steel tools not be used. A trial and error process of increasing feed rate through the machine and slower spindle speeds for the tool can be used to attempt to achieve an acceptable finish on the work article.
Typically, the fabricator (machine operator) is given a thermoplastic material that may have been designed for a particular engineering application and filled with a reinforcing material, for example, glass fiber. In these situations, the thermoplastic material is typically chosen for the end use and not necessarily to facilitate machining of the plastic. Furthermore, the work piece is typically held in a fixed position and the machining tool moved to produce the desired shape. As a result, the machining process can be slow and the parts small compared to the size of parts that would typically be needed for use as wood substitute in building applications. For these reasons, the engineering thermoplastics used for these machined applications can be too costly for application as wood substitutes in building applications.
Recently, wood filled thermoplastic composites have been introduced as wood substitutes in decking, cladding and simple trim applications that do not require intricate shapes. These composites are typically produced by profile extrusion and, depending on the material and the difficulty of producing the shape, may be expensive. Also, many desirable shapes for shaped articles may not be successfully produced with the dimensional consistency required when using desirable inexpensive thermoplastic materials. In other applications, profile extrusion may be too expensive to provide parts with the low dimensional variability (tight tolerances) preferred for use in applications such as support boards for extruded aluminum door sills or thresholds. Moreover, there are a number of challenges for these wood plastic composite parts for many wood replacement applications, including, but not limited to, aesthetics, toughness (elastic and flexural modulus), and machinability.
Extruded plastic composite materials are now in use as deck boards and the like. However, these also can have drawbacks as a raw material for intricate shaped articles. Products made from extruded cellulose filled polyethylene, polypropylene or polyvinyl chloride, often referred to as wood plastic composites, are readily available as deck boards. However, they have undesirable sensitivity to moisture, and in some cases, exposure to sunlight. Many of these can suffer from “blow-out” when screws are inserted into the ends of extruded boards; the ends can tend to “blow out”, (a chunk of material breaks away), because the material may be brittle. The plastic composite materials may also be very heavy relative to wood and can add substantial weight and cost to the application. Use of foaming to decrease the weight and cost can make the plastic composite, especially wood plastic composites, even more prone to “blow out”. Finally, wood has very low coefficient of thermal expansion (CTE), approximately one-tenth the CTE of typical wood plastic composite materials. In some building exterior applications that are constrained, for example, in a door threshold support, this can lead to bending or bowing of the constrained piece.
Filled oriented polymer composition (OPC) articles produced from solid state die drawing are also known for use as wood substitutes. A major challenge for solid state die drawing is that intricate shapes and dimensions are not readily drawn into the final part. Inside corners with sharp features, channels with sharp features, undercuts, etc., can be very difficult to achieve with die drawing. Thus, for shaped articles for building applications, for example, decorative trim pieces, coving, brick molding, door jambs, etc., machining (planer/molder/routers) is necessary to achieve the desirable shapes and dimensions. Another challenge encountered in the machining or cutting of OPC articles is surface fibrillation, which can result from the “tear-out” of oriented polymer strands during various types of cutting operations on an oriented polymer composition article. Fibrillation of the surface can occur when an OPC article is cut as with a saw or is machined by any of a number of wood working techniques, for example, routering, molding and the like. “Tear-out” is when, during the machining process, a fibril or cluster of fibrils is produced at the cut or machined surface which, when pulled, can peel or “chip” away from neighboring aligned polymer chains leaving a gap in the surface.
U.S. Pat. No. 5,204,045 to Courval et al. addresses surface fibrillation in OPCs by creating a skin of low orientation. Another example, U.S. Pub. No. 2009/0155534, to O'Brien et al., describes an alternate method of addressing surface fibrillation by applying a surface treatment which results in a deoriented surface layer and reduced fibrillation when the article surface is cut or scratched. The surface layer is from 80 to 400 microns in thickness and is of lower orientation than a 100 microns thick layer adjacent to the deoriented surface layer. However, in both cases, surface fibrillation is only addressed within a predetermined depth from the surface of the article.